The importance of the body content and serum concentration of potassium to control blood pressure remains controversial. Epidemiological data suggest a role for potassium depletion as a co-factor in the development and severity of hypertension, while dietary potassium inversely correlates with blood pressure [8-10]. In animal models, an acute increase in serum potassium concentration produces vasodilatation mediated by the vascular endothelium; the opposite effect is observed if it decreases [11,12]. In haemodialysis, the extent of the difference between serum potassium and the potassium concentration in the dialysis fluid is directly correlated to an increase in blood pressure at the end of the dialysis session, producing what has been named "rebound hypertension" [1]. In this same study no significant changes in blood pressure were found during the dialysis.